define(function () {
    
    const isServer = false;
    const SPECIAL_CHARS_REGEXP = /([\:\-\_]+(.))/g;
    const MOZ_HACK_REGEXP = /^moz([A-Z])/;
    const ieVersion = isServer ? 0 : Number(document.documentMode);
    
    /* istanbul ignore next */
    const trim = function(string) {
        return (string || '').replace(/^[\s\uFEFF]+|[\s\uFEFF]+$/g, '');
    };
    /* istanbul ignore next */
    const camelCase = function(name) {
        return name.replace(SPECIAL_CHARS_REGEXP, function(_, separator, letter, offset) {
            return offset ? letter.toUpperCase() : letter;
        }).replace(MOZ_HACK_REGEXP, 'Moz$1');
    };
    
    /* istanbul ignore next */
    const on = (function() {
        if (!isServer && document.addEventListener) {
            return function(element, event, handler) {
                if (element && event && handler) {
                    element.addEventListener(event, handler, false);
                }
            };
        } else {
            return function(element, event, handler) {
                if (element && event && handler) {
                    element.attachEvent('on' + event, handler);
                }
            };
        }
    })();
    
    /* istanbul ignore next */
    const off = (function() {
        if (!isServer && document.removeEventListener) {
            return function(element, event, handler) {
                if (element && event) {
                    element.removeEventListener(event, handler, false);
                }
            };
        } else {
            return function(element, event, handler) {
                if (element && event) {
                    element.detachEvent('on' + event, handler);
                }
            };
        }
    })();
    
    /* istanbul ignore next */
    const once = function(el, event, fn) {
        var listener = function() {
            if (fn) {
                fn.apply(this, arguments);
            }
            off(el, event, listener);
        };
        on(el, event, listener);
    };
    
    /* istanbul ignore next */
    function hasClass(el, cls) {
        if (!el || !cls) return false;
        if (cls.indexOf(' ') !== -1) throw new Error('className should not contain space.');
        if (el.classList) {
            return el.classList.contains(cls);
        } else {
            return (' ' + el.className + ' ').indexOf(' ' + cls + ' ') > -1;
        }
    };
    
    /* istanbul ignore next */
    function addClass(el, cls) {
        if (!el) return;
        var curClass = el.className;
        var classes = (cls || '').split(' ');
        
        for (var i = 0, j = classes.length; i < j; i++) {
            var clsName = classes[i];
            if (!clsName) continue;
            
            if (el.classList) {
                el.classList.add(clsName);
            } else {
                if (!hasClass(el, clsName)) {
                    curClass += ' ' + clsName;
                }
            }
        }
        if (!el.classList) {
            el.className = curClass;
        }
    };
    
    /* istanbul ignore next */
    function removeClass(el, cls) {
        if (!el || !cls) return;
        var classes = cls.split(' ');
        var curClass = ' ' + el.className + ' ';
        
        for (var i = 0, j = classes.length; i < j; i++) {
            var clsName = classes[i];
            if (!clsName) continue;
            
            if (el.classList) {
                el.classList.remove(clsName);
            } else {
                if (hasClass(el, clsName)) {
                    curClass = curClass.replace(' ' + clsName + ' ', ' ');
                }
            }
        }
        if (!el.classList) {
            el.className = trim(curClass);
        }
    };
    
    /* istanbul ignore next */
    const getStyle = ieVersion < 9 ? function(element, styleName) {
        if (isServer) return;
        if (!element || !styleName) return null;
        styleName = camelCase(styleName);
        if (styleName === 'float') {
            styleName = 'styleFloat';
        }
        try {
            switch (styleName) {
                case 'opacity':
                    try {
                        return element.filters.item('alpha').opacity / 100;
                    } catch (e) {
                        return 1.0;
                    }
                default:
                    return (element.style[styleName] || element.currentStyle ? element.currentStyle[styleName] : null);
            }
        } catch (e) {
            return element.style[styleName];
        }
    } : function(element, styleName) {
        if (isServer) return;
        if (!element || !styleName) return null;
        styleName = camelCase(styleName);
        if (styleName === 'float') {
            styleName = 'cssFloat';
        }
        try {
            var computed = document.defaultView.getComputedStyle(element, '');
            return element.style[styleName] || computed ? computed[styleName] : null;
        } catch (e) {
            return element.style[styleName];
        }
    };
    
    /* istanbul ignore next */
    function setStyle(element, styleName, value) {
        if (!element || !styleName) return;
        
        if (typeof styleName === 'object') {
            for (var prop in styleName) {
                if (styleName.hasOwnProperty(prop)) {
                    setStyle(element, prop, styleName[prop]);
                }
            }
        } else {
            styleName = camelCase(styleName);
            if (styleName === 'opacity' && ieVersion < 9) {
                element.style.filter = isNaN(value) ? '' : 'alpha(opacity=' + value * 100 + ')';
            } else {
                element.style[styleName] = value;
            }
        }
    };
    
    return {
        on,
        off,
        once,
        addClass,
        removeClass,
        setStyle,
        getStyle
    }
});